Mg(NO.sub.3).sub.2.6H.sub.2 O/NH.sub.4 NO.sub.3 forms a eutectic mixture which phase changes at about 52.degree. C. (126.degree. F.) releasing or alternately absorbing a substantial quantity of latent heat of fusion. Such properties make the material an excellent choice as a heat storage medium for use in solar energy space heating applications. In order to improve the material for such uses, however, there exists a need to develop additives or a series of additives to suppress the inherent supercooling properties of the Mg(NO.sub.3).sub.2.6H.sub.2 O/NH.sub.4 NO.sub.3 liquid phase, and particularly additives which are highly effective in small amounts.